In isostatic or semi-isostatic compaction of powder into solid bodies, a mould is utilized which is at least partially defined by an elastomeric wall, which defines a mould cavity for filling with the powder. According to the so-called dry-bag technique, the elastomeric wall is an expandable chamber wall in a hydraulic press containing a hydraulic chamber behind the wall. The mould thus forms a part of the press itself. Accordingly, the press is inactive when the mould is being filled with powder and when the compacted product is removed. Since the press itself is burdened with large costs, it is extremely important to utilize it as intensively as possible.
According to the so-called wet-bag technique, the pressing chamber of the press is wet, the mould being removable from the press for being filled with powder and being relieved of the compacted product outside the press. However, there is the disadvantage that the hydraulic liquid of the press contaminates the mould and surroundings and can be contaminated by the powder.
It has therefore also been proposed to combine the wet-bag technique and the dry-bag technique. For this the pressing chamber is implemented as a dry cylindrical hydraulic pressure chamber having one end wall constituting an end wall of the mould. The mould is then made as a unit formed by a closure for the open end of the press and an elastomeric jacket which substantially isostatically translates the pressure from the expandable pressing chamber wall to the enclosed powder. It is then essential that the mould jacket consists of an elastomer capable of substantially isostatically translating the pressure to the enclosed powder. However, the elastomeric jacket of the mould must also have satisfactory stability of shape during filling with the powder or granules. This is usually achieved by manufacturing the elastomeric jacket with a comparatively large wall thickness from a material with a relatively large shore hardness. The result of this is that large elastic return forces are developed by the jacket when pressure is unloaded at the termination of the pressing operation. Apart from the possibility of these forces causing damage to the compacted body, we have also found that a thick and hard elastomeric jacket for the moulding tool can result in form deviations of the compacted product. It is possible that these form deviations are the result of the elastomeric jacket, at the end of the compacting operation, nonuniformly returning to its original shape in different partial areas, such that the contact pressure remains in some partial areas while it has been released from others in the compacted body.
It is indeed known, e.g. from the Swedish published specification No. 361 008, to avoid injurious rebound forces of the elastomeric jacket at the end of the compacting operation by making the jacket thin-walled and supporting it with the aid of a porous elastomeric foam through which the hydraulic fluid of the press can pass for coming into contact with the impermeable elastomeric jacket. However, this known art has nothing to offer in respect of the inventive technique, according to which the mould is to be arranged as a unit for coaction with a dry hydraulic press chamber.
One object of the invention is therefore to provide a press tool unit intended for coaction with a dry pressing chamber in a hydraulic press for semi-isostatic compaction of a powder enclosed in the mould cavity, in which the drawbacks of form deviation of the kind mentioned have been avoided.
A further object is to provide a mould unit of the kind mentioned, which may contain a plurality of form cavities for simultaneous compaction of a plurality of products during one working stroke of the press.